How Auto Transmissions Work

Identification

As of 1939, car transmissions saw an improvement when the automatic transmission system became available. Going from manual to automatic meant the driver had less work to do as the system itself took on most of the work of shifting, or changing gears. Instead of the clutch work being driver-controlled, the transmission itself determines when to shift through a series of coordinated gears and mechanisms. Besides managing shifting, the system is also responsible for keeping the engine from working too hard as the car's speed changes, or when towing a heavy load. This being so, a key function of an automatic system is to modulate between engine torque and gear speeds. A series of gears work on and off to adapt to different speeds and conditions. This coordinated effort between the gears and the engine happens in ratios, or gear ratios. A special gearbox sits at the center of this process.

Function

A car's motor is connected to an automatic transmission through the gearbox. The gearbox connects to the motor drive shaft, also known as the engine's output shaft. Actual gear ratios are determined inside the gearbox, where certain gears are set in motion while other gears are made to remain stationary. Various gear sizes, positions and configurations make the most of their interlocking teeth to allow the engine's power to be directed in different areas. Gear movements are responsible for the direction the car moves in, synchronizing wheel rotations, the car's weight distribution, and wheel-speed rotations. The size differences and positions of the gears allow engine power to be redistributed for different purposes, whether to move in reverse, tow a trailer, drive on the highway or drive in the city.

Effects

A good example of how the gearbox converts, or redistributes, engine power is when a car is in a low gear. Lots of engine power can be applied and the car still moves slowly. However, its traction power is greater, whereas a car cruising down the highway has minimum traction with ample speed. In each case, the gearbox has set a certain set of gears in motion in order to convert the engine's power in a particular way. Engine-fuel efficiency is best when the engine is working comfortably and the car's speed is fast, as occurs in highway driving. What the gearbox does is leverage gear sizes, positions and speeds so that they make the most of the engine's power. The engine is not necessarily working harder; rather, the transmission is working smarter.

References

• Automatic Transmission Workings

Resources

• Automatic-transmission problems